That burner systems are used in a variety of applications such as building heating systems, industrial processes, power generation, etc. goes without saying. Typically, newer burner systems use microprocessor-based controls because of the reliability, economy, flexibility, efficiency, and capability microprocessors provide. The microprocessor receives numerous signals indicating various conditions relating to burner operation and provides control signals to the burner system which cause each of the various burner system functions to be initiated and terminated properly. The microprocessor also receives demand signals arising externally which specify when the burner system should operate and perhaps the level of combustion required as well. When heat is needed, the microprocessor issues a number of commands to the burner system which cause the burner system to pass through a sequence of operating phases which prepare the burner system for the run phase which denotes combustion of fuel flowing through the main valve. Just before the run phase, there is a pilot phase, during which the pilot valve is open and the pilot light is burning. The pilot light is used to light the main valve fuel as the burner system moves into the run phase. During the run and pilot phases, the microprocessor provides a standby signal having a first state and during other phases of operation the standby signal has a second state, the term "standby" in this context denoting that there is no flame within the combustion chamber.
It is of supreme importance that burner system operation be managed safely, and one of the key aspects of this requirement is that fuel be supplied to the burner system's combustion chamber only when a flame is actually present. A flame sensor is employed to assure that flame is present whenever either of the fuel valves are open. If the flame sensor should indicate absence of flame while the standby signal has its second state, then any open fuel valve is closed immediately to prevent unburned fuel from accumulating.
A common type of flame sensor used for electronic burner system controls senses the ultraviolet radiation from the combustion process and provides an electronic flame signal having an analog value increasing and decreasing as the radiation impinging on the sensor increases or decreases. This analog value may take a number of different forms such as a voltage or current level or the duration between level changes in the signal. In a particular system now available from the assignee of this application, a specific level of the value encoded in the sensor signal is defined as a threshold level indicating presence of flame. In this embodiment, current level has been chosen to forms the flame signal with 0.8 .mu.amp. as the threshold level. Flame sensor current greater than this amount is interpreted as indicating presence of flame. Current less than this amount is interpreted as absence of flame.
Because of the nature of the sensor and the environment within combustion chamber, there is a tendency for their performance to deteriorate or degrade over a period of time. Because the deterioration tends to increase the signal level when no flame is actually present, there is the potential for the unsafe condition to arise of flame indicated by the flame signal when in fact no flame is present. In fact, however, procedures have been developed for assuring that flame is not incorrectly indicated as present. These procedures can detect when the signal provided by the flame sensor has finally become unreliable.
Flame sensor operation can deteriorate or become marginal for a number of reasons such as degradation of the sensor's internal elements, or dust and moisture which affects operation. The ability to detect both the pilot flame and the main flame at the appropriate times in the burner startup sequence requires precise initial alignment of the flame sensor and competent maintenance thereafter. When flame sensor operation deteriorates in this way for any reason, nuisance shutdowns may occur because of failure to detect the presence of a flame which is actually present.
This deterioration of a flame sensor is a gradual process which eventually results in its signal shifting out of the ranges specified for presence or absence of flame when the particular condition exists. This deterioration requires sensor replacement or maintenance when the erroneous signal causes the control system to unnecessarily shut down the burner system. Delaying replacement or maintenance may cause these nuisance shutdowns to occur at a time when the repair will be expensive or inconvenient. Accordingly, it would be useful to determine sensor deterioration before actual sensor signal failure occurs and while flame sensor operation is still safe.